Effect Of Green Liquor Pretreatment On The Lignin Structure And Enzymatic Saccharification Of Agricultural Residues

Lignocellulose has natural resistance to biological degradation because of itsmorphological structure and chemical composition. Pretreatment is required to obtainhigh conversion of polysaccharides to fermentable sugars during bioethanolproduction. A novel pretreatment with the concept of repurposing an old kraft pulpmill for bioethanol production based on green liquor (Na2CO3+Na2S) pretreatmentwas developed in North Carolina state University, USA. This process uses onlyproven technology and equipment currently used in a kraft pulp mill and has severaladditional advantages such as high sugar recovery and concentration, no inhibitivesubstances produced, as compared to acid-based pretreatment methods. Therefore,this dissertation was mainly focused on the following three issues:1) the effect of GLpretreatment on agricultural straw,2) the change of lignin structure during GLpretreatment, and3) the interaction between lignin structure change and enzymaticdigestibility.In Chapter2and3, green liquor (GL) pretreatment was carried out onagricultural straw for investigating the effect of GL pretreatment on delignificationand sugar conversion in sequence enzymatic hydrolysis. The optimized pretreatmentconditions were8%total titratable alkali (TTA) charge,40%sulfidity,140°C for cornstover and4%TTA charge,20%sulfidity and140°C for rice straw. After GLpretreatment,71.9%and73.1%of sugar in untreated corn stover and rice straw,respectively, were converted into fermentable sugars, using20FPU/g pulp of enzyme in the subsequent enzyme hydrolysis. Meanwhile, GL pretreatment exhibited thereasonable delignification selectivity for improving enzymatic digestibility. The resultsuggested GL pretreatment is a practicable method for corn stover rice straw toenhance enzymatic saccharification for bioethanol production.In chapter4, rice straw was pretreated by sodium sulfite–formaldehyde (SF) forimproving enzymatic saccharification. And72.7%of sugar in untreated rice straw wasobtained at an enzyme loading of20FPU/g-substrate after the raw material pretreatedwith12%sodium sulfite at160°C. The development of SF process also with theconcept of reuse an old sulfite pulp mill for bioethanol production. However, toachieve the similar sugar yield, the condition of SF pretreatment was more severe thanthat of GL pretreatment. And the chemical recovery of SF process was not as good asGL process.In chapter5, a novel process of dissolution and regeneration was carried out onwheat straw to modify the isolation method of cellulolytic enzyme lignin (CEL), andfor the enhancement of carbohydrate removal and lignin dissolution with a short timeof ball milling. The characteristics of isolated lignin from internode and leaf wererather different. The leaf lignin exhibited a higher condensation and had less syringyl(S) unit than internode. The internode lignin contained more β-O-4linkage that wasmainly composed by erythro form than that of leaf. Compared to CEL, little structuresand chemical compositions change was observed in RCEL. Therefore, RCEL can beused for structural and compositional analysis of lignin, as well.In chapter6, to examine why green liquor (GL) pretreatment of lignocellulosicmaterials effectively facilitates enzymatic saccharification, dimeric β-O-4type ligninmodel compounds were reacted in several alkaline solutions including a modelsolution of green liquor. The results suggest that β-O-4bonds present in lignocellulosic materials are effectively cleaved in a GL pretreatment. It was alsosuggested that HS-contributes to the β-O-4bond cleavage of the non-phenolic modelcompound synergistically with CO32-under GL pretreatment conditions. A newreaction was found, and the reaction resultants (Vanillin and veratrylaldehyde) wereconfirmed, although the mechanism of the new reaction was not clear.In chapter7, green liquor (GL, Na2CO3+Na2S) and sodium carbonate (SC)pretreatments were comparatively investigated to understand the lignin removal andstructural changes of wheat straw internode and leaf (sheath included) in pretreatmentprocess. Compared with SC pretreatment, GL pretreatment resulted in betterdelignification selectivity and higher final sugar yield after enzymatic hydrolysis. Thepretreated leaf was easier to be hydrolyzed than pretreated internode, which mightcorrelate with the difference in lignin structure.GL pretreatment can efficiently remove lignin from agricultural straw, modifylignin structure in pretreated soild and improve digestibility during enzymatichydrolysis. GL pretreatment is a feasible method to enhance enzymaticsaccharification during bioethanol production from agricultural residues.